PHP实现事件机制的方法

<?php
/**
* 事件
*/
class event {
 private $callbacks = array();
 private $holder;
 function __construct() {
  $bt = debug_backtrace();
  if (count($bt) < 2) {
   $this->holder = null;
   return;
  }
  $this->holder = &$bt[1]['object'];
 }
 function attach() {
  $args = func_get_args();
  switch (count($args)) {
   case 1:
    if (is_callable($args[0])) {
     $this->callbacks[]= $args[0];
     return;
    }
    break;
   case 2:
    if (is_object($args[0]) && is_string($args[1])) {
     $this->callbacks[]= array(&$args[0], $args[1]);
    }
    return;
   default:
    return;
  }
 }
 function notify() {
  $bt = debug_backtrace();
  if ($this->holder && 
    ((count($bt) >= 2 && $bt[count($bt) - 1]['object'] !== $this->holder)
    || (count($bt) < 2))) {
   throw(new exception('notify can only be called in holder'));
  }
  foreach ($this->callbacks as $callback) {
   $args = func_get_args();
   call_user_func_array($callback, $args);
  }
 }
}